Corrosion of a metal structure immersed in an electrolyte results from flow of local current through the electrolyte between localized anodic and cathodic portions of the structure surface, the corrosion occurring at the anodic surface portions. A familiar example is the corrosion of iron or iron alloy structures immersed in water. Prevention of such corrosion by cathodic protection involves passing direct current (applied by a suitable current source) through the electrolyte from one or more anodes immersed therein to the metal structure to be protected, which is connected to the negative terminal of the current source to constitute the cathode of the system. The purpose of providing this applied current is to establish and maintain at the structure surface, including the localized anodic portions thereof, a negative polarization potential effective to prevent the corrosion-producing local current flow.
Since the effectiveness of a cathodic protection system is dependent on maintenance of a sufficient electro-negative polarization potential at the structure to be protected, it is desirable to control the operation of the cathodic protection system, as by adjustment of the applied current flow, in response to changes in the structure potential. Such control may be accomplished by measuring the structure potential and actuating appropriate control means in accordance with the potential measurement to vary the current supply from the direct current source so as to maintain the polarization potential at a desired value.
If the electrolyte in which the protected structure is immersed has a sufficiently low resistivity, the polarization potential of the structure may conveniently be determined by immersing in the electrolyte a suitable non-polarized reference electrode of fixed potential and directly measuring the difference of potential between this reference electrode and the structure. In such circumstance, the potential drop through the electrolyte between the reference electrode and the structure is so small as to be negligible in its effect on control of the system, i.e., variations in the potential difference between the electrode and structure are due substantially only to variations in structure polarization potential and hence provide effective system control.
However, when the resistivity of the electrolyte is high (e.g., greater than about 1,000 ohm-centimeters, as in the case of electrolytes such as potable waters) the potential difference measured between the reference electrode and the structure includes a significant potential drop resulting from the flow of cathodic protection current through the resistive electrolyte between the electrode and structure. This potential drop varies with changes in the applied current and/or in the resistivity of the electrolyte. As a result, in operation in a high-resistivity electrolyte, utilizing measurement of potential difference between a reference electrode and the protected structure to control the operation of a cathodic protection system as heretofore known, the system responds to variations in electrolyte resistivity or applied current density as well as to variations in the polarization potential of the protected structure. This is undesirable, since the potential drop resulting from applied current flow through the electrolyte has little or no relation to the effectiveness of control of local current flow at the metal surface to be protected; accordingly, for proper regulation of a cathodic protection system to maintain a desired polarization potential at the structure surface, the determination or sensing of the polarization potential should be accomplished in a manner that is independent of the latter potential drop, i.e., which effectively eliminates variations in such potential drop as a control factor in the system.
While the undesired potential drop component of the polarization potential measurement can be very substantially reduced by placing the reference electrode on the protected surface, such arrangement presents difficulties in that the reference electrode then senses the potential of only a very limited area of the structure surface, and in addition the electrode may partially shield the surface from the applied protective current.
Difficulties similar to those described above are encountered in controlling anodic protection systems operating for passivation of metal surfaces (e.g., such as stainless steel surfaces) immersed in highly resistive electrolytes. In an anodic passivation system, the structure to be protected is connected to the positive terminal of a direct current source, the negative terminal of which is connected to auxiliary electrode means immersed in the electrolyte. To effect and maintain passivation of the structure surface, it is necessary that the structure potential (determined by comparison with standard reference electrode means) be controlled within a limited range of values, by regulation of the current source; if the structure potential departs from this range, the anodic protection operation may actually enhance the rate of structure corrosion. As in the case of cathodic protection, it is desirable to eliminate, from the measurement of structure potential used for system control, variables due to electrolyte resistivity and current density, which may be introduced in the measurement if the electrolyte resistivity is sufficiently high to provide an appreciable potential drop between the measuring reference electrode means and the structure through the electrolyte.